## Summary

Branch-cut step to start testing the **2.13** release in tutorials.
Mirrors #3831 ("Enable nightly build to test 2.12").

- **`.github/workflows/build-tutorials-nightly.yml`**: re-enable the
`push` on `main` trigger so the nightly tutorials build runs against the
2.13 RC binaries.
- **`.jenkins/build.sh`**: pin `torch==2.13.0` (from the `test` channel,
cu130) on the `USE_NIGHTLY` path and the commented reference line;
update the `2.12 -> 2.13` comment.

`.ci/docker/requirements.txt` is intentionally left at `torch==2.12` —
that gets bumped to 2.13 at the later "move to stable binaries" step
(the #3891 equivalent), once 2.13 is released.

## Release playbook reference (2.12 -> 2.13)
| Step | 2.12 PR | 2.13 |
|------|---------|------|
| Enable nightly build to test the RC | #3831 | **this PR** |
| Move to stable binaries (requirements.txt) | #3891 | follow-up at
release |

## Test plan
- `bash -n .jenkins/build.sh` passes.
- After merge, the nightly tutorials build runs on `main` and installs
`torch==2.13.0` from `https://download.pytorch.org/whl/test/cu130`
(2.13.0 test wheels are published).